Converted 85mm f/1.2 breech to EOS (Canon30D)

Started Aug 23, 2008 | Discussions
jrmcvey Forum Member • Posts: 62
Converted 85mm f/1.2 breech to EOS (Canon30D)

After seeing all the beautiful bokeh coming out of this forum, I decided to buy a 85mm f/1.2 Aspherical from ebay and try a "lens" conversion. I had already done this once with an older (and cheaper) 55mm f/1.2 FL lens, so I felt it was not too risky to try it on a $500 lens. Attached are a few photos for your review. Included are some of the machining operations on my lathe to remove enough of the lens body and FD mount, so I could replace it with a thinned EOS t-mount and still hit infinity focus. (FD lenses have 42mm of back focus -- EOS cameras require 44mm, so that 2mm has to come off the lens body.) I haven't mounted a focus confirmation chip yet, so these early shots are manually focused on my 30D with the stock focusing screen.

Some of you may ask "why" ... it's because I enjoyed the challenge and love working in the machine shop ... but don't ask me to do one for you. There were times I was pulling my hair out ... like when one of the f/stop ball bearings rolled off somewhere in the kitchen ...

The flower images are "out of camera" with no processing other than a downsize for this posting.


 jrmcvey's gear list:jrmcvey's gear list
Panasonic Lumix DMC-G6 Panasonic Lumix DC-G9 Panasonic Lumix G Vario 7-14mm F4 ASPH Panasonic Lumix G 20mm F1.7 ASPH Panasonic Lumix G Vario 45-200mm F4-5.6 OIS +6 more
ScottyNV Veteran Member • Posts: 3,889

very well done, indeed!

I would expect the camera to expose properly in Av and M modes, and those would be the only ones I'd likely use with such a conversion. How is the optical quality? I may consider this kind of operation on a couple of FD lenses when I get back from vacation in two weeks.. You did a much neater job than I did when I modded my EF-S 10-22 to fit my 5D (it does and now works quite well as a 12-22mm 'stupid wide' lens)

My XT IS Full Frame -- APS-C/FF of course!
So is my 5D -- 35mm/FF

andrewD2 Veteran Member • Posts: 9,510
hit 1/8000s limit on samples

Interesting conversion, should be a useful lens. At f1.2 outside, even at ISO100 you can run into the 1/8000s fastest shutter speed limit and overexpose.


Arcus Regular Member • Posts: 102
Re: Converted 85mm f/1.2 breech to EOS (Canon30D)

I cannot imagine all the trouble that you done to convert this lens to use with the 30-D. Having he said this, I think you will have a good time with thia lens.

genotypewriter Veteran Member • Posts: 4,846
Bravo! Question...

It's fantastic to see this kind of work... photographers are usually a bad bunch of consumers but seeing someone putting work in to making something unique gives great feelings. I hope you enjoy your lens!

I have a couple of questions...

Did anything related to MFD and infinity focus change because of this modification?

I'm interested in modifying lenses and was wondering if you could give a quick general overview of what you did to adapt this lens to EF? E.g. how you chose the parts and how you decided where to cut etc.?

Thanks in advance!


OP jrmcvey Forum Member • Posts: 62
Re: 85mm f/1.2 convert and Bravo! Question...

Thanks to those that responded to my post. Here are some additional comments:

I've been quite happy with the optical quality so far including color balance and saturation. Of course I'm using an APS-C sensor, so I'm using the "sweet spot" of the lens. The samples I posted were shot wide open at f/1.2 with no post-processing so you can judge for yourself.

As mentioned by another poster, non-computerized lenses such as this should be used in Av or M modes. My 30D seems to meter quite accurately in Av mode with manual lenses.

Any wide-open fast aperture lens will challenge the camera to have a fast enough shutter speed to prevent over exposure outdoors on a bright day. The "sunny 16" rule suggests this lens at f/1.2 would need a shutter speed of something like 1/16000 sec to correctly expose at ISO 100. Canon's new EOS 85mm f/1.2L II is no different in this regard. The Canon 50mm f/1.4 needs 1/12800 sec!

Converting the lens did not affect the MFD or infinity focus. The focus distance scale is still accurate. On a FF camera the DOF scale would also still be accurate and the overall illumination (MFD) is not materially changed.

The lens conversion was quite a challenge and it never goes as easily as hoped. I always select the older "breech" style lenses to convert rather than the (I believe) more complicated New-FD style with the internally rotating mount. Each lens seems to have a unique structure to it. In fact I have learned that Canon has several diameters of the basic silver breech locking ring.

I begin the conversion by removing the FD mount and measuring it's thickness. In this case the FD mount was 2.5mm thick. Remember the EOS mount must be 2mm thinner to preserve infinity focus. If it was possible to get a strong EOS lens mount (ring) that was only 0.5mm thick then it could be attached in place of the FD mount and the job would be mostly complete. But a strong EOS mount is at least 1mm thick, so then you are forced to remove metal from the lens barrel itself to help make up that 2mm back-focus reduction. This requires disassembly of the helicoids and you must make careful measurements and documentation so that reassembly goes correctly.

If this was not enough, the f/stop is dependent on linkage contained in the FD mount, so once that is removed, a bit of ingenuity is required to relink the f/stop ring to the iris mechanism.

About 8 hours of work for the mechanically inclined hobby-machinist and you're done. Just don't let the little f/stop click-stop balls roll under the fridge!


 jrmcvey's gear list:jrmcvey's gear list
Panasonic Lumix DMC-G6 Panasonic Lumix DC-G9 Panasonic Lumix G Vario 7-14mm F4 ASPH Panasonic Lumix G 20mm F1.7 ASPH Panasonic Lumix G Vario 45-200mm F4-5.6 OIS +6 more
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